Cirrus Logic AN364 - Manual (Page 17)

AN364
AN364REV3 17
The BSTAUX pin and FBAUX pin currents must be limited to less than 1mA. A series resistor of at least 22 k 
must be used to limit the current.
Step 22) Overvoltage Protection
Output open circuit protection and output overvoltage protection (OVP) are implemented by monitoring the
output voltage through the flyback transformer auxiliary winding. During switching time T2, the voltage across
the flyback transformer T1 auxiliary winding is representative of the output voltage using a turns ratio
relationship. The flyback auxiliary winding voltage is applied to the FBAUX pin. If the voltage on the FBAUX
pin exceeds threshold V
OVP(th)
, which is set to 1.25V, a fault condition occurs. The IC output is disabled and
the controller attempts to restart after one second.
Since the flyback auxiliary winding does not supply V
DD
, the auxiliary winding circuit has fewer design
constraints. A flyback auxiliary output voltage of 10V to 20V during switching period T2 is appropriate. The
resistive divider between the flyback auxiliary winding and the FBAUX pin must be sized to produce 1.25V
when an overvoltage fault occurs at the desired load. The FBAUX pin current must be limited to less than 1mA.
Step 23) External Overtemperature Protection
The external overtemperature protection (eOTP) pin is used to implement overtemperature protection using a
negative temperature coefficient (NTC) thermistor. The total resistance on the eOTP pin is converted to an 8-
bit digital ‘CODE’ (which gives an indication of the temperature) using a digital feedback loop, adjusting the
current (I
CONNECT
) into the NTC and series resistor R
S
to maintain a constant reference voltage of 1.25V
(V
CONNECT(th)
). Figure 9 illustrates the functional block diagram when connecting an optional NTC
temperature sensor to the eOTP circuit.
Current I
CONNECT
is generated from an 8-bit controlled current source with a full-scale current of 80 A. See
Equation 24:
When the loop is in equilibrium, the voltage on the eOTP pin fluctuates around V
CONNECT(th)
. The digital
‘CODE’ output by the ADC is used to generate I
CONNECT
. In normal operating mode, the I
CONNECT
current is
updated once every seventh half line-cycle by a single ±LSB step. See Equation 25:
CS1610/11
+
-
I
CONNE CT
V
CONNE CT (th)
Comp_Out
eOTP
Control
eOTP
R
S
C
NTC
NTC
V
DD
10
(Optional )
Figure 9. eOTP Functional Diagram
I
CONNECT
V
CONNECT th  
R
------------------------------------- = [Eq. 24]
CODE
I
CONNECT
2
N
--------------------------- 
V
CONNECT th  
R
NTC
R
S
+
------------------------------------- = [Eq. 25]